Micromechanical function will be understood as an arbitrary active function, for example a sensor function, or passive function, for example a printed circuit trace function.
Although applicable to arbitrary micromechanical components and structure, in particular to sensors and actuators, the present invention and its underlying problem will be explained with reference to a micromechanical component, for example an acceleration sensor, which is manufacturable in the technology of silicon surface micromachining.
Monolithically integrated, surface-micromachined inertial sensors with movable structures affixed to a chip in an unprotected manner (analog devices) are generally known. These designs cause an increased expenditure of time and energy during handling and packaging.
These problems can be avoided using a sensor having an evaluation circuit on a separate chip; in this context, for example, the surface-micromachined structures are covered by a second cap wafer. This type of packaging composes a high portion of the cost of a surface-micromachined acceleration sensor. This cost results from the large surface required for the sealing surface between the cap wafer and the sensor wafer and because of the complex patterning (2-3 masks, bulk micromachining) of the cap wafer.
The evaluation circuit is implemented on a second chip and connected to the sensor element via wire bonding. This, in turn, creates the necessity for the sensor elements to be sized such that the parasitic effects developing due to the parasites in the lead wires and bonding wires are negligible and that they no longer have any dominant influence on the sensor function. In addition, flip-chip techniques are ruled out because of parasitic effects.
Such sensors could use considerably less surface for the micromechanics if the evaluation circuit were situated on the same Si chip and the sensitive electrodes could be connected with only low parasities.
German Published Patent Application No. 195 37 814 describes the structure of a functional layer system and a method for the hermetically packaging of surface-micromachined sensors. Explained in this context is the manufacture of the sensor structure using known technological methods. The mentioned hermetical packaging is effected using a separate cap wafer which is made of silicon and patterned using complex patterning processes such as KHO etching. The cap wafer is affixed to the substrate featuring the sensor (sensor wafer) using a glass solder (seal glass). For this purpose, a broad bonding frame is required around each sensor chip to ensure sufficient adhesion and tightness of the cap. This considerably reduces the number of sensor chips per sensor wafer. Due to the large space required and the complex manufacture of the cap wafer, considerable costs are attributable to the sensor packaging.
German Published Patent Application No. 43 41 271 describes a micromechanical acceleration sensor whose components are composed partly of monocrystalline material and partly of polycrystalline material. For manufacturing this micromechanical acceleration sensor, an epitaxial reactor is used. A starting layer made of LPCVD polysilicon is used for determining the regions where polycrystalline silicon is intended to grow during the epitaxial process.